Respiratory syncytial virus is the most common cause of lower respiratory tract disease in young children worldwide. The F glycoprotein of this virus is one of the important targets of a protective immune response; however, immunity is incomplete and repeated infections occur. Previously, a detailed map of the neutralization epitopes was developed based on the ability of a panel of murine anti-F monoclonal antibodies to neutralize 14 monoclonal antibody resistant mutant viruses of the A2 strain. The reactivity of the F monoclonal antibodies with the panel of F MARMs in ELISA was also assessed in order to corroborate the operational map previously developed and to demonstrate a direct correlation between resistance to neutralization and absence of antibody binding. In most cases MARMs did not bind the selecting antibody and verified the idea that escape from neutralization was due to specific mutations in the contact residues within the F epitope. In addition, 25 neutralizing and non-neutralizing F MAbs have been biotinylated and used in competitive binding studies to identify at least 6 antigenic sites on the F glycoprotein. These biotinylated MAbs were also used to develop an antigenic site blocking assay in order to analyze the immune response to specific regions on RSV-F following infection or immunization. Preliminary studies have shown that post- infection human and cotton rat sera that have similar neutralization titers, vary in the repetoire of their anti-F antibody response. During the coming year additional studies will be done to evaluate individual immune responses to the F glycoprotein following natural infection or immunization. As an extension of this work, the reactivity of F MAbs with synthetic peptides will be evaluated in order to develop an epitope specific site-directed ELISA assay.